Little-known fault poses quake risk.

Many residents of the San Francisco Bay area immediately recognize the names San Andreas and Hayward as the two major faults framing the Bay. But recent geologic studies suggest that a more obscure fault to the north may pose the greatest immediate risk to Bay area residents.

Researchers from the U.S. Geological Survey (USGS) in Menlo Park, Calif., recognized several years ago that this fracture, the Rodgers Creek fault, warranted concern. They have now gathered evidence that the fault may be nearing a magnitude 7 earthquake. "Of all the segments in the area, this fault is probably closest to failure," says USGS geologist David P. Schwartz. He and his co-workers found evidence of three prior earthquakes when they dug trenches across the Rodgers Creek fault between San Pablo Bay and Santa Rosa. Carbon-14 dating suggests that the shocks occurred every 300 years or so, with the last earthquake dating to approximately 1700.

If the Rodgers Creek fault does rupture regularly, it should produce another shock in the next few decades, Schwartz says. He warns, however, that the limited data currently available make it difficult to tell how regularly the fault generates quakes.